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also require sea ice. They spend their time either resting on top of it (and watching out for polar bears), or swimming beneath it looking for Arctic cod. The Arctic cod lurks under and along the edges of the ice, watching out for ringed seals while chasing amphipods, copepods, and krill. Those little creatures in turn graze on tiny flagellates and diatoms that grow on the underside of the ice, and also bloom profusely in the water alongside its melting edge. This entire food chain—from microscopic phytoplankton to a thousand-pound polar bear—is inextricably linked to the presence of sea ice. Walruses, bearded seals, and other species also use sea ice, though none so specifically as do polar bears, ringed seals, and Arctic cod.

Despite growing evidence of stress (like bear cannibalism), none of these species is in immediate risk of extinction. But there is little question that if the summertime sea-ice fades completely, then these amazing creatures will fade right along with it. Government scientists, in a report to aid the Bush administration’s decision on the proposed Endangered Species Act listing, estimate that two-thirds of the world’s polar bears will be gone by 2050.303

From these and other indications worldwide, climate change is forcing a massive ecological reorganization of the planet, with both extinctions and expansions now under way. Depending on the emission scenario used, one model projects that anywhere from 15% to 37% of the world’s species will be committed to climate-change extinction by 2050.304 If these numbers hold true, they are devastating—roughly comparable to the impacts of deforestation and other direct forms of habitat loss. When combined with all the other species extinctions since the last ice age, they will mark the sixth great mass extinction on Earth—and the first since the Cretaceous-Tertiary extinction that ended the dinosaurs some sixty-five million years ago.

The mechanisms for climate-change extinction are many. Amphibians and wetland species are especially vulnerable to droughts. As temperatures rise, polar and alpine species have literally nowhere left to go once pushed off the brink of the northernmost coast or highest mountain peak. A less direct mechanism is the decoupling of codependent species within a food web (called “match-mismatch” by ecologists) when their respective phenological cycles fall out of whack. Imagine birds migrating to their accustomed nesting area only to find that the caterpillar hatch they were planning to gorge on has already come and gone, for example. Another is that warmer temperatures tend to enable insect pests, invasive species, disease, and robust “generalized” species (like rats and raccoons) to outcompete specialized ones. Yet another is that the projected rate of climate change is so rapid that some sedentary species (like trees) may not be able to relocate quickly enough, or their escaping climatic comfort zone will shift to a place incompatible for other reasons, like terrain or soil. Some climates, especially in alpine and polar areas, will simply cease to exist. By century’s end, under a high carbon emissions scenario, 10%-48% of the world’s land surface is projected to “lose” its extant climate completely, and 12%-39% will develop new “novel” climates that don’t exist in the world today (mostly in the tropics and subtropics).305 These changes will have powerful impacts on world ecosystems and could even render some local conservation efforts obsolete. Finally, because ecosystems and food webs have so many complex interconnections, there will be rippling effects we don’t yet know about. All of this is piled on top of an ongoing raft of familiar ecological threats, including habitat destruction, invasive species, and pollution.

Compared with other places, habitat loss and pollution are less severe in Alaska, northern Canada, the Nordic countries, and eastern Russia, where vast boreal forests, tundra, and mountains hold some of the wildest and least-disturbed places left on Earth.306 However, northern ecosystems also have far simpler food